J/ApJ/448/179 HST photometry in R136 (Hunter+ 1995)
The intermediate stellar mass population in R136 determined from Hubble Space
Telescope planetary camera 2 images
Hunter D.A., Shaya E.J., Holtzman J.A., Light R.M., Earl J., Oneil Jr.
<Astrophys. J. 448, 179 (1995)>
=1995ApJ...448..179H 1995ApJ...448..179H
ADC_Keywords: Magellanic Clouds ; Photometry ; Clusters, open
Keywords: Hertzsprung-Russell diagram - Magellanic Clouds -
open clusters and associations: individual (R136) - stars: evolution -
stars: luminosity function, mass function
Abstract:
We have analyzed Hubble Space Telescope (HST) images of the compact,
luminous star cluster R136 in the LMC that were taken with the
refurbished HST and new Wide Field/Planetary Camera. These images
allow us to examine the stellar population in a region of unusually
intense star formation at a scale of 0.01pc. We have detected stars to
23.5 in F555W and have quantified the stellar population to an M_555.0
of 0.9 or a mass of 2.8M☉. Comparisons of HR diagrams with
isochrones that were constructed for the HST flight filter system from
theoretical stellar evolutionary tracks reveal massive stars, a main
sequence to at least 2.8M☉, and stars with M_555.0≥0.5 still on
pre-main sequence tracks. The average stellar population is fit with a
3-4Myr isochrone. Contrary to expectations from star formation models,
however, the formation period for the massive stars and lower mass
stars appear to largely overlap. We have measured the IMF for stars
2.8-15M☉ in three annuli from 0.5-4.7pc from the center of the
cluster. The slopes of the IMF in all three annuli are the same within
the uncertainties, thus, showing no evidence for mass segregation
beyond 0.5pc. Furthermore, the combined IMF slope, -1.22±0.06, is
close to a normal Salpeter IMF. The lower mass limit must be lower
than the limits of our measurements: ≤2.8M☉ beyond 0.5pc and
≤7M☉ within 0.1pc. This is contrary to some predictions that
the lower mass limit could be as high as 10M☉ in regions of
intense massive star formation. Integrated properties of R136 are
consistent with its being comparable to a rather small globular
cluster when such clusters were the same age as R136. From the surface
brightness profile, an upper limit for core radius of 0.02pc is set.
Within a radius of 0.4pc we estimate that there have been roughly 20
crossing times and relaxation should be well along. Within 0.5pc
crowding prevents us from detecting the intermediate mass population,
but there is a hint of an excess of stars brighter than M_555.0=-5 and
of a deficit in the highest mass stars between 0.6pc and 1.2pc. This
would be consistent with dynamical segregation.
Objects:
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RA (2000) DE Designation(s)
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05 38 42.8 -69 06 03 R 136 = RMC 136 = HD 38268
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 126 3623 The photometry of stars in the R136 region
(completed with astrometry in April 2016)
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 6 I6 --- Star [1/3623] Star identification number
8- 13 F6.1 pix Xpos [0/800] X pixel coordinate (1)
15- 20 F6.1 pix Ypos [0/800] Yixel coordinate (1)
22- 27 F6.2 mag Umag [10/22]?=100.00 HST/WFPC2 F336W magnitude
29- 34 F6.2 mag e_Umag [0.01/0.6]?=100.00 F336W magnitude uncertainty
36- 41 F6.2 mag Vmag [12/25]?=100.00 HST/WFPC2 F555W magnitude
43- 48 F6.2 mag e_Vmag [0.01/1.4]?=100.00 F555W magnitude uncertainty
50- 55 F6.2 mag Imag [11/24]?=100.00 HST/WFPC2 F814W magnitude
57- 62 F6.2 mag e_Imag [0.01/0.6]?=100.00 F814W magnitude uncertainty
64- 71 F8.2 mag U-V ?=100.00 F336W-F555W color
73- 80 F8.2 mag e_U-V [0.02/0.6]?=100.00 F336W-F555W color uncertainty
82- 89 F8.2 mag V-I ?=100.00 F555W-F814W color
91- 98 F8.2 mag e_V-I [0.02/1.5]?=100.00 F555W-F814W color uncertainty
102-103 I2 h RAh [5] Right Ascension J2000 (hours) (2)
105-106 I2 min RAm [38] Right Ascension J2000 (minutes) (2)
108-113 F6.3 s RAs Right Ascension J2000 (seconds) (2)
115 A1 --- DE- Declination J2000 (sign) (2)
116-117 I2 deg DEd [69] Declination J2000 (degrees) (2)
119-120 I2 arcmin DEm [5/6] Declination J2000 (minutes) (2)
122-126 F5.2 arcsec DEs Declination J2000 (seconds) (2)
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Note (1): the X,Y coordinates are those in the short exposure F555W image,
and the coordinate system is indicated in Figure 2 of the printed paper.
Note (2): accurate positions were supplied in April 2016 by B. Skiff
(Lowell Obs.); the astrometry accuracy is estimated about ±0.1arcsec,
the internal error being about 0.02arcsec (0.3pix). More details about
the reduction are given below in the "Astrometry" section below.
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Astrometry of R136 stars, by Brian A. Skiff, Lowell Obs (2016-04-08):
A straightforward quadratic fit yielded mean residuals of 0.3 pixels
in each axis at an image-scale of 0".045/pixel. Thus the internal
errors are only a few hundredths of an arcsecond.
The coordinates for the reference stars have come solely from my
reduction of the Selman et al 1999 study (J/A+A/341/98/table2),
where I used a number of UCAC2 and 2MASS stars for the reference
frame. Although they are tied very closely to the ICRF system, the
two catalogues inevitably have some "softness" in this extremely
crowded region. Clearly the small internal errors indicate the
Selman catalogue is pretty good.
Comparison with not-too-crowded stars appearing in the excellent IRSF
(II/288), in the Parker 1992 survey (II/187, also similarly reduced by
me), and other sources with less well-defined astrometry suggests the
overall external adjustment is about ± 0.1" rms. However, because of
the extreme crowding I leave the results to 0.01" precision, if only
so that cases like R136a1 and a2 (stars 3 and 5 of the table) can be
distinguished.
History:
* 29-Jan-1996: from AAS CD-ROM series, Volume 5, 1995
prepared by Lee Brotzman [ADS] on 10-Oct-1995
* 08-Apr-2016: accurate positions added, from B. Skiff [Lowell Obs.]
(End) [CDS] 29-Jan-1996